Many routine locomotor activities require that muscles undergo lengthening contractions in order to absorb energy and decelerate the body. These eccentric contractions can damage myofibrils and often result in severe delayed-onset muscle soreness. While initial bouts of eccentric activity often result in muscle damage, repeated exposure to eccentric training has been shown to greatly decrease a muscle's susceptibility. I propose to examine the role of the cytoskeletal protein, titin, in resisting eccentric muscle damage. The broad objectives of this proposal are to understand 1) how short-term active modulation of titin stiffness functions to limit damage during an eccentric exercise, and 2) how long-term shifts in titin isoforms following repeated exposure to training reduce susceptibility to eccentric damage. The first aim of the proposal is to determine how in vivo muscle length changes and muscle activity patterns during eccentric exercise transiently alter the passive stiffness of muscle in order to resist eccentric muscle damage. To achieve this aim I will use an experimental design in which the in vivo lengths and activation patterns used during downhill (eccentric) and uphill (concentric) running are directly imposed on the same muscle in situ in order to quantify transient shifts in a muscle's passive stiffness during eccentric and concentric exercise. The second specific aim of this proposal is to determine whether repeated exposure to eccentric training results in an adaptive shift in the expression of titin isoforms, which increase muscle stiffness and reduce susceptibility to damage. To achieve this aim a subset of individuals will be exposed to downhill training for 8 weeks. Following the training protocol muscle biopsies will be taken from trained and untrained individuals in order to compare titin expression patterns using an SDS-PAGE analysis. In addition passive stiffness and resistance to eccentric damage will be compared in trained and untrained muscles using an in situ muscle preparation and histological methods. Relevance- It is well established that routine eccentric activities can result in particularly severe muscle damage in individuals suffering from Duchenne and Becker muscular dystrophies as well as in muscles atrophied from extended periods of immobilization or neuromuscular degeneration. This increased propensity for muscle damage can hinder the efficacy of rehabilitative therapies and result in a cycle of weakness and discomfort that discourage the continued utilization of physical therapies. A better understanding of muscles adaptive response can aid in the design of more effective rehabilitative strategies.